1. Technical Field
This invention relates to a digital electronic control apparatus providing a pulse width modulated output signal for the control of at least one electronic control element of a hydraulic system. The present invention relates specifically to a digital electronic control supplying a pulse width modulated output signal for the control of an electric solenoid which is used for actuating a hydraulic valve which, in turn, is adapted to adjust hydraulic characteristics, like for instance the pressure and/or the flow of a hydraulic medium like hydraulic oil.
2. Description the Prior Art
Digital electronic control apparatus of the above mentioned type are already known and in use. For instance, such apparatus includes a current controller which supplies an output signal for maintaining a control element in a position determined by a desired value signal.
Typically, the digital output signal (control or controlling value) of the current (I) controller is transformed into a pulse width modulated signal (PWM-signal), the duty cycle of which is proportional to the output signal value of the current controller. The electronic control can also be designed as an exclusively current control chain.
The PWM-signal is used for switching an electronic switch that supplies, for instance, the coil of a solenoid with voltage so that the solenoid is switched on and off with constant frequency in accordance with the duty cycle. As a result thereof an energizing current and an induced current, respectively, occur alternately, and a respective proportional voltage is derived from a metering resistance and is supplied as the actual value signal (middle value of current=current middle value) to the current controller.
DE 37 07 442 Al discloses a prior art control electronic apparatus.
The known PWM-control electronic apparatus are adjusted to the impedance of the control element so as to be controlled such that a 100% use is made of the digital resolution of the control value. In case, the electronic control is used to control a control element having a different, for instance a lower impedance, then the digital resolution of the control value can not be obtained up to 100%. For instance, if the electronic control is designed for a control element having an impedance of 24 ohms then a control element having a impedance of 12 ohms can use the control value only in the amount of 50%, inasmuch as in this case twice the current is reached.
Thus, a control electronic apparatus is desirable which provides, for different impedance values, a high resolution of the electronic control values.